Make library AOT compatible

[giggio]

Right now we are getting warnings, like:

/home/user/.nuget/packages/cloudnative.cloudevents.systemtextjson/2.7.1/lib/netstandard2.1/CloudNative.CloudEvents.SystemTextJson.dll : warning IL2104: Assembly 'CloudNative.CloudEvents.SystemTextJson' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries [/app/consumer.csproj]
/home/user/.nuget/packages/cloudnative.cloudevents.systemtextjson/2.7.1/lib/netstandard2.1/CloudNative.CloudEvents.SystemTextJson.dll : warning IL3053: Assembly 'CloudNative.CloudEvents.SystemTextJson' produced AOT analysis warnings. [/app/consumer.csproj]

[jskeet]

I'm afraid I don't have enough experience with AOT (or time to get up to speed) to work on this. I'd happily review a PR to address it, however.

[giggio]

@jskeet I just added a PR for it, it was a lot of work, I need revision to make sure everything is ok.